Semiconductor image sensor

ABSTRACT

A semiconductor image sensor which has photocells arranged in a matrix form is miniaturized and integrated with high density, thereby to increase its light amplification factor and operating speed. To this end, each photocell is formed by a static induction transistor which has a pair of main electrodes, a channel region formed between the main electrodes and a capacitor connected between a control region serving as a photocell and one of the row lines.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor image sensor which is characterized in that cells are each formed by using a static induction transistor as a light detecting and switching element.

A conventional semiconductor image sensor has each cell formed by a light detecting diode and a switching MOS transistor, and possesses the defect of low sensitivity because light detection is carried out by the diode. Since the MOS transistor is used for the switching operation, switching noise is larger than the signal of the light and removal of the noise is difficult. For the reason mentioned above, the use of the cell formed by the light detecting diode and the MOS transistor imposes limitations on the integration density from the viewpoint of sensitivity.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor image sensor of the one cell-one transistor type which employs a static induction transistor of high photosensitivity for both light detection and switching operations.

Another object of the present invention is to provide a semiconductor image sensor which is suitable for miniaturization and for high integration density.

Another object of the present invention is to provide a semiconductor image sensor of a large light amplification factor.

Still another object of the present invention is to provide a semiconductor image sensor which permits high-speed readout.

In the semiconductor image sensor of the present invention, a matrix is formed by a plurality of row lines and a plurality of column lines and, at each intersection of the matrix, a static induction transistor is provided which has a first main electrode connected to one of the column lines and the other main electrode connected in common to the other main electrodes of the other static induction transistors, a channel region disposed between the main electrodes and a capacitor connected between a control region serving as a photocell and one of the row lines. The first main electrode of a respective switching transistor is connected to each one of the column lines and another main electrode of all these switching transistors are connected in common, and connected via a load resistor to a power source. The gate or base of each switching transistor is connected to a video line selecting circuit and the gate of each photocell is connected via the capacitor to a readout address circuit.

The semiconductor image sensor of the present invention has such features as follows:

(1) the semiconductor image sensor of the present invention has each photocell formed by one transistor and one capacitor, and hence is simple in construction; and

(2) the light amplifying action is large and resistant to noise.

Thus the semiconductor image sensor of the present invention is simple in construction, has a light amplifying action, possesses characterisitics unobtainable with conventional semiconductor devices, and hence is of great industrial value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a diagram showing a photocell for use in the semiconductor image sensor of the present invention;

FIG. 1(b) is a diagram explanatory of the construction of the semiconductor image sensor of the present invention;

FIGS. 1(c) and (d) are cross-sectional views of the principal parts of the photocells for use in the semiconductor image sensor of the present invention;

FIG. 1(e) is a diagram explanatory of the operation of the semiconductor image sensor of the present invention;

FIG. 1(f) is a graph showing an example of a dynamic characteristic of this invention device;

FIGS. 2(a) and (b) are cross-sectional views of the principal parts of photocell with a shielding gate;

FIG. 3(a) is a cross-sectional view of a photocell matrix in the semiconductor image sensor of the present invention in which a number of photo cells are arranged;

FIG. 3(b) is a top plan view of the photocell matrix;

FIGS. 3(c) and (d) are diagrams showing the arrangement of the semiconductor image sensor of the present invention which performs a two-dimensional image sensing operation; and

FIGS. 4 (a) and (b) are cross-sectional views of the principal parts of other examples of photocells for use in the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1(a) and (b) illustrate embodiments of a transistor of a cell portion and the circuit structure of one cell portion of the semiconductor image sensor of the present invention, respectively.

FIG. 1(a) shows an n-channel static induction transistor which has its gate connected to a capacitor and has what is called normally OFF characteristic such that current is difficult to flow even if the gate voltage is zero. Voltage is applied between the drain and the source and, when light input 30 is applied, holes of electron-hole pairs created in the channel are stored in the gate region. That is, when the transistor is put in the ON state by applying voltage between the drain and the source and a forward voltage between the gate and the source, a drain-to-source current flows corresponding to the number of holes stored in the gate region, carrying out the amplification of light. The light amplification factor becomes as high as 10³ or more; namely, the sensitivity of the static induction transistor is one or more orders of magnitude higher than the sensitivity of a conventional phototransister formed by a bipolar transistor. The capacitor connected to the gate is for cutting off direct current and for storing the light signal.

FIG. 1(b) illustrates the basic structure of the present invention. Reference numeral 31 indicates the static induction transistor shown in FIG. 1 (a), 22 a switching transistor, 23 a load resistor, 24 a video voltage source, 25 a lead (a video line) between the switching transistor 22 and the load resistor 23, φ_(S) a pulse voltage which is applied to the gate of the switching transistor 22, and φ_(G) a pulse voltage which is applied to the gate of the transistor 31.

Holes optically excited by the light input 30 flow into the gate region of the phototransistor 31 to write therein the light signal. The pulse voltage φ_(S) is provided to the gate of the transistor 22 to conduct it to apply the voltage of the video voltage source 24 to the photo transistor 31. At this time, the pulse voltage φ_(G) is not applied. When the pulse voltage φ_(G) is provided to conduct the static induction transistor 31, a drain current is caused to flow corresponding to the light input, producing a light output signal from an output terminal 26.

The light output at the output terminal 26 varies with the intensity of the light input 30 and a characteristic of a wide dynamic range could be obtained.

FIG. 1(c) illustrates an embodiment of the semiconductor image sensor of the present invention, showing in section the semiconductor device of the cell and a circuit structure necessary for operation. Reference numeral 1 indicates an n⁺ substrate of silicon, 2 a high resistance n⁻ layer (or an intrinsic semiconductor region and a region which will ultimately form a channel), 3 a high impurity density n⁺ region which will ultimately form a source region, 4 a high impurity density p⁺ region which is so shaped as not to close the channel region and will ultimately form a control gate, 6 an insulator, such as SiO, 7, 8 and 10 gate, source and drain electrodes, and 9 an SiO₂ film.

Reference numeral 20 denotes a read address circuit which generates the readout gate pulse voltage φ_(G), and 21 a circuit which generates the video line selecting pulse voltage φ_(S) for controlling the switching transistor 22.

The control gate 4, the insulator 6 and the gate electrode 7 constitute the capacitor which is connected to the gate. The insulator 6 need not be limited specifically to SiO₂ but may also be Si₃ N₄, Al₂ O₃, tantalum oxide or a composite film thereof.

The operation of this structure is the same as the device of FIG. 1(b).

For obtaining the static induction transistor, the impurity density of the n⁻ region of the channel is selected lower than about 1×10¹⁶ cm⁻³ and the impurity densities of the gate, source and drain regions are selected higher than around 1×10¹⁸ m⁻³. In order that no drain current may flow even if the gate electrode is zero, the sizes and the impurity densities of respective regions are selected so that the regions between adjacent gate regions and the channel region may be depleted by a diffusion potential alone. It is needless to say that this can easily be achieved by increasing the thickness of the gate to reduce the gate spacing. For the light amplification, care should be taken not to introduce a dislocation, a defect and so forth in the crystal in each manufacturing step; for example, in the case of diffusing boron for the formation of the p⁺ gate, the group IV atom is used to compensate for lattice distortions for preventing them. In order to prevent easy recombination of the optically excited electron-hole pairs, it is necessary that the n⁻ region of the channel be long-lived and, to this end, gettering for heavy metals is carried out in the final stage of the manufacturing process to thereby increase the lifetime of the channel region. The read address circuit 20 and the video line selecting circuit 21 can each be formed by a shift register.

FIG. 1(d) is a modified form of the structure of FIG. 1(c), in which the source and drain electrodes are reversed and a substrate electrode 10 is used as the source.

In the case of integrating the read address circuit 20, the video line selecting circuit 21, the switching transistor 22, the load resistor 23, the lead 25 and the output terminal 26 as peripheral circuits on the same substrate, a more covenient one of the circuit structures of FIGS. 1(c) and (d) may be selected. The switching transistor 22 as the peripheral circuit may be the normally OFF type static induction transistor as is the case with the cell, or a MOS transistor. The read address circuit 20 and the video line selecting circuit 21 may also be of the same circuit structure as an ordinary random access memory (RAM).

FIG. 1(e) is a diagram showing the operation of the semiconductor image sensor of the present invention. The switching transistor is the normally OFF type static induction transistor. Reference character φ_(G) indicates a readout gate pulse voltage, and φ_(S) a video line selecting pulse voltage. It is seen from FIG. 1(e) that when light is irradiated to write light information and the video line selecting pulse voltage φ_(G) is provided, the column line of the cell to be read out is selected and, only when the pulse voltages φ_(S) and φ_(G) are applied simultaneously, is an output signal produced. At this time, the pulse voltage φ_(S) is 2 V, the pulse voltage φ_(G) is 2 V and video voltage is 6.10 V, but even if the video voltage is 2 V or so, the semiconductor image sensor of the present invention operates. It has been found that the semiconductor image sensor of the present invention operates on a very low voltage. Further, the readout speed is less than 100 nsec per cell and, accordingly, the semiconductor image sensor of the present invention can be made of large-capacity. Since the normally OFF type static induction transistor is used, the power sources for the pulse voltages φ_(S) and φ_(G) the drain power source may be of the same polarity.

FIG. 1(f) shows an example of the dynamic characteristic caused by light. The size of the photo cell is 50 μm×55 μm. The time for irradiation by light is 25 msec. The applied pulse voltages φ_(S) and φ_(G) are both 1 V, the video voltage is 6.1 V, a load resistance R_(L) is 1 KΩ and the capacitance of the capacitor connected to the gate is about 5 pF. In view of the relation between the intensity of incident light and the output voltage, it is seen that the semiconductor image sensor of the present invention is very highly sensitive. In particular, it is seen that linearity on the side of low-intensity light is two or more orders of magnitude over that on the side of high-intensity light. Similar results were also obtained in the case of using, as the transistor of the photocell, such a static induction transistor in which no drain current flows when a gate voltage is applied.

FIGS. 2(a) and (b) illustrate other embodiments of the present invention, which are identical in construction with the embodiments of FIGS. 1(c) and (d) except that the photocell has two gate regions. On the control gate 4 is formed a capacitor made up of the insulator 6 and the gate electrode 7. Reference numeral 13 indicates a shielding gate, which is formed to surround the control gate 4 and the n⁺ source region 3. The control gate 4 and the shielding gate 13 constitute a potential barrier in the channel. Electrons of the electronhole pairs created by the light input 30 flow into the ground potential, but the holes are stored in the gate region 4 and the floating gate 13. In the drawings only one photocell is shown, but when a number of photocells are formed, the p⁺ shielding gate 13 has the function of isolating each cell by a depletion layer from adjacent the one. A certain potential may sometimes be applied between the floating gate 13 and the source, or the floating gate may also be made equipotential to the source.

FIG. 2(a) shows an embodiment in which the n⁺ region 3 is used as the source region, and FIG. 2(b) an embodiment in which the n⁺ region 3 is used as the drain region.

FIGS. 3(a) through (d) illustrate embodiments of the present invention as applied to a semiconductor image sensor for image processing.

FIG. 3(a) is a cross-sectional view of the photocell portion taken on the line A--A' in FIG. 3(b) (the dimensions not being the same). Reference numeral 1 indicates an n⁺ substrate of silicon, 2 a high resistance n⁺ layer (or an intrinsic semiconductor region), 3 a high impurity density n⁺ region which is the source region (or the drain region), 4 a control gate region formed by a high impurity density p⁺ region, and 13 a floating gate. These regions are identical with those in the embodiment of FIG. 2. On the wafer surface are disposed a control gate electrode 8 and an electrode 16 of the n⁺ region 3 which will ultimately form the source or drain. Reference numeral 14 denotes a thin insulator layer as of SiO, and a capacitor is formed between the gate electrode 8 and the control gate region 4. Reference numeral 9 indicates an SiO film, and 15 phosphorus-doped polysilicon which interconnects the source regions of respective photocells. Reference numeral 16 denotes a phosphorus-doped phosphosilicate glass (PSG) layer, which is provided for insulating the gate electrode 8 and the polysilicon 15 of the source region.

FIG. 3(b) is a top plan view of FIG. 3(a). Reference numeral 8 indicates aluminum lines of the gate electrodes. The n⁺ regions and the gate regions are indicated by the broken lines. The surface is covered with the PSG film. The control p⁺ gate region and the thin insulating film 14 for the capacitor (FIG. 3(a)) underlie the gate electrode 8.

FIG. 3(c) illustrates an embodiment of the present invention which is formed by an arrangement 32 of static induction transistors of FIGS. 3(a) and (b) in a matrix form and which is capable of two-dimensional image detection. Reference numeral 22 indicates a switching transistor for video line selection, which is an example of the normally OFF type static induction transistor. Reference numeral 21 denotes a video line selecting circuit which provides the pulse voltage φ_(S), 20 a read address circuit which provides the pulse voltage φ_(G), 23 a load resistor, and 24 a video voltage source.

By irradiation of the light input 30, the video line selecting circuit and the read address circuit derive in succession at an output terminal 26 signals of photocells which form the matrix. By sequentially transmitting the outputs at the output terminal 26 to a display circuit, an image output can be obtained.

The device of FIG. 3(d) is identical with the device of FIG. 3(c) except that the n⁺ region 3 is used as the drain and the n⁺ substrate is used as the source region in FIG. 3(a). It is also possible to integrate the matrix of the photocells and the peripheral circuits. When it is desired to obtain a color display, a semiconductor image sensor for color display can be obtained by separating the photocell array 32 through using color filters, for instance, by providing red (R), green (g) and blue (B) cells to take out signals of R, G and B.

FIGS. 4(a) through (c) illustrate other embodiments of the present invention, showing isolation of each photocell portion.

In FIGS. 4(a) and (b), the photocells in the embodiments of FIGS. 1(d) and 2(b) are isolated by insulating material layers 40. The insulating material layers are formed of SiO or the like.

FIG. 4(c) uses a p substrate 41 for isolating the photocells in the embodiment of FIG. 2(b) by a p-n junction. Reference numeral 42 indicates a p or p⁺ region, which is connected to the p substrate 41. Reference numeral 43 denotes n⁺ region, which is the source or drain region. Reference numeral 44 designates a source electrode which connects the source region 43 to the wafer surface by an n region. The p (p⁺) region 41 and the source region 43 may also be connected together and via the source electrode as illustrated.

It is needless to say that the read address circuit and the video line selecting circuit can be integrated on the same substrate, along with the photocell matrix through utilization of known integrated circuit techniques other than the abovesaid isolation method.

With the present invention, it is possible to obtain a semiconductor image sensor of the type that each cell is formed by one transistor and one capacitor through using the static induction transistor.

While the embodiments have been described in connection with the case where the transistor used is an n-channel one, it is needless to say that a p-channel transistor can also be used. Also it is possible to employ an SIT whose channel is reverse in conductivity type. The gate may also be a Schottky barrier gate or MOS (MIS) gate. The material need not be limited specifically to silicon but may also be germanium and the group III-V compound semiconductors. It is needless to say that wiring on the surface of the photocell array may be effected by conventional digital memory techniques, such as double wiring of an aluminum line or the like. 

I claim:
 1. A semiconductor image sensor comprising:a matrix corresponding to intersections of a plurality of row lines and a plurality of column lines on a substrate; a static induction transistor photocell provided for each said intersection, each said photocell includinga first main electrode connected to one of the column lines, a second main electrode connected in common to the second main electrodes of the other photocells, a channel region disposed between the first and second main electrodes, a control region, and a respective capacitor connected between each said control region and a respective row line; a respective switching transistor for each said column line, each includinga control electrode; a first main electrode connected to the respective column line, and a second main electrode connected in common to the second main electrodes of the other switching transistors; and a respective video line selecting circuit for each said column line, each connected to the control electrode of the respective switching transistor; and a respective read address circuit for each said row line, connected to the gate electrode of each respective photocell via the respective capacitor.
 2. A semiconductor image sensor according to claim 1, wherein the matrix with the static induction type photocells, the switching transistors, the video line selecting circuits, the read address circuits and the load resistor are integrated on said substrate.
 3. A semiconductor image sensor according to claim 1, wherein each said video line selecting circuit and read address circuit are formed by a respective shift register and are integrated on said substrate. 